Acetabulum endoprosthesis and head

ABSTRACT

An implantation method is described which involves the successive introduction of projecting multilateral needles into the spongy bone of a joint. The needles are symmetrically spaced on the terminal surfaces of the endoprosthesis up to a resistance edge on one portion of an endoprosthesis and up to a resistance surface on a second portion of the endoprosthesis. The remaining free area between the projecting multilateral needles is filled up to the terminal surfaces in the &#34;biological silence&#34; by osteoblasts. The endoprosthesis also includes a glenoid cavity and a head which have round terminal surfaces with the projecting multilateral needles placed thereon. The projecting multilateral needles have different lengths and mutually parallel axes which are perpendicular to the planes in which the round resistance edge of the glenoid cavity and the resistance plane of the head are located.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an endoprosthesis to apply in bonesurgery, without using surgical cement, and more particularly to aprosthetic acetabulum and head.

2. Brief Description of the Related Art

There are known endoprosthesis the connection of which with the bonetakes place by the growing into pores of proper dimensions, intoindentations and other irregularities on the surface of theendoprosthesis, fixed on the spot where it is mounted with the help ofsurgical cement, or mechanically. In order to stop bone loss, it hasbeen proposed to use an endoprosthesis joined with the bone by pushingsurgical cement into the marrow cavity for example,polymethylmethacrylate cement, or with the help of coarse threads. Theseendoprostheses suffer from a very small bonding surface formed byingrowth of spongy bone.

Furthermore, the joining of an endoprosthesis to the bone by surgicalcement has some defects. The main defect is that the material weakens asit ages, which causes it to fail. Consequently, irritation from thefailing cement causes a separation from the bone and this, in turn,causes loosening of the endoprosthesis and the bone's atrophy.

DE 3443109 A1 describes a prosthesis in which the prosthetic implantshave more secure fastenings by applying conic irregularities which stickout of the terminal surface of the endoprosthesis and which are directedagainst the 'joining surface of the bone. The conic irregularities arepressed into the spongy bone and cause an increase in the contactsurface which is favourable to fixation of the prosthesis to the bone bybone ingrowth. An imperfection of this solution is the fact that thereis no possibility to optimally increase the contact surface of theendoprosthesis with the bone because the conic irregularities are spacedapart on a plane surface. However, the plane contact surface, because ofit's shape, renders impossible ingrowth of spongy bone in a shapesimilar to the natural form, which would assure a good absorption of thejoint's forces.

FR A 2 519 545 describes a prosthesis in which the acetabulum has atotal spherical form and also, on the top of the endoprosthesis,includes conic irregularities which stick out of the boundary surface ofthe endoprosthesis and which are directed against the joining surface ofthe pelvic bone; this structure, however, is not adapted to thephysiological structure of the bone. For this reason it can not beimplanted due to the fact that the "bottom" of the acetabulum in thebone of the pelvis is flat, hard and made out of a thin cortex in thepelvic bone. Therefore, the needle on the top of the acetabulum cap cancause a perforation of the bone, which causes a weakening and finally adamage thereto: a bleeding of the blood vessels inside the pelvis.

Another deficiency of the device described in FR A 2 519 545 is theinsertion of the endoprosthesis, which occurs at the same time asimplantation of the needles in the cap. Furthermore, insertion of theendoprosthesis in such a form, in which all of the needles have the samelength and extend over the highest point of the external surface of theacetabulum, is imperfect because of the anatomical structure of thepelvis bone. Such an installation of the endoprosthesis makes impossiblebone ingrowth between the sharply ended needles, and therefore causes aweak connection of the bone with the endoprosthesis, because theconnection is purely mechanical and causes unfavourable transmission ofthe biomechanical forces of the hip joints.

FR A 2 686 503 describes a prosthesis in which the endoprosthesisfacilitates only a partial reconstruction of the femoral head surface,which has been removed because of necrosis. The imperfection of thisinvention is that the endoprosthesis does not permit resection of thewhole femoral head surface. Additionally, implantation of theendoprosthesis requires a special hole for the keel 5, which complicatesthe surgery and diminished the adhesion. Because the endoprosthesisaccording to this solution diminished the adhesive interface surface, itis not stable for a non-cemented solution.

SUMMARY OF THE INVENTION

The aim of the invention is the elimination of these imperfections bycreating an endoprosthesis, thanks to which there is a simple, safe anddurable union with the bone tissue of the cortex as well as thetrabecular bone, ensuring at the same time the possibility of rebuildingthe regenerated bone in a form similar to the natural and original bone.

This aim has been achieved thanks to this invention according to whichan endoprosthesis includes an acetabulum and a head, while the bearingsurfaces are located on round surfaces which include multilateralprojecting needles, preferably in the form of a pyramid with parallelaxes and different lengths. The edges of the bases of adjacent needlescontact each other, and their axes are substantially perpendicular tothe surface in which lies the bearing edge of the acetabulum as well asthe bearing surface of the head. The total interface bearing surfacesare advantageously more than seven times larger than the joint surfaceof the acetabulum and the head, and the needles advantageously are sizedso that the ratio of the half diagonal to the height of the pyramid ismore than one to five. Peaks of the projecting multilateral pyramidneedles of the acetabular cap do not extend beyond the circular boundarysurface determines by the edge lying on the plane perpendicular to theacetabular axis, however the head has a bearing surface in annular formwith an outer diameter less than a diameter of the round bowl whichconstitutes a spherical cap of the external surface of the head.

Still other objects, features, and attendant advantages of the presentinvention will become apparent to those skilled in the art from areading of the following detailed description of embodiments constructedin accordance therewith, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention of the present application will now be described in moredetail with reference to preferred embodiments of the apparatus andmethod, given only by way of example, and with reference to theaccompanying drawings, in which:

FIG. 1 schematically illustrates the acetabulum of the endoprosthesis incross-section; and

FIG. 2 schematically illustrates the head of the endoprosthesis incross-section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawing figures, like reference numerals designateidentical or corresponding elements throughout the several figures.

The endoprosthesis of the joints is composed by the acetabulum (1) andthe head (2). The acetabulum (1) has an external spherical boundarysurface (3) which is equipped with multilateral needles (4). A circularsurface (5) is determined by the edge (6) lying in the perpendicularplane to the longitudinal axis of the acetabulum. The multilateralneedles (4) are preferably of pyramid form (not shown in the figures)which have common edges with the bases of neighbouring needles (4) andhave differing lengths. The length of the multilateral needles (4)measured from the base on the boundary surface (3) determines aconcentric line to the boundary surface (3), the theoretical sphericalsurface (7) which crosses the peaks of a part of the multilateralneedles (4) as well as the plane in which lies the circular surface (5),so the peaks of the multilateral needles (4) do not exceed the plane inwhich lies the edge (6), whilst the ratio of the half diagonal of theneedle bases to the height of the needle pyramid is at least one tofive, and the total surface area of the multilateral needles (4) is morethan at least seven times more than that of surface (9).

The acetabulum (1) is equipped with a pan (8) to place the head (2)which constitutes a part of the spherical cap of the external surface(9). The head (2) has a annular bearing surface (10) lying below thetransverse axis of the head (2) in a perpendicular plane to theconcentric line, and a spherical boundary surface (11). Boundary surface(11) defines a spherical cap, designated by a ray which equals a half ofthe diameter from the two inner edges of the bearing surface (10),beginning from the bisection point of the concentric axis of the head(2) in which the bearing surface (10) is located. On the sphericalboundary surface (11) there are multilateral needles (12) and a centralmultilateral needle (13) with parallel axes to each other, and themultilateral central needle is coincident with the axis of the head (2).The multilateral needles (12) have common edges at their bases (notshown in figures) with neighbouring multilateral needles (12), and havediffering lengths. The tip of the central multilateral needle (13) lieson the sphere which includes the surface (9). The remaining lengths ofthe multilateral needles (12) are determined by the distance betweentheir base on the boundary surface (11) to the tips which lie on thetheoretical surface of the caps (14) with a radius which is equal to thehalf of the distance between the axis of the multilateral needles (12)and the axis of the multilateral central needle (13) carried out fromthe point which lies between the axes on the straight line whichconnects the peaks of opposite extreme needles (12), so that theremaining needles are longer than the multilateral ones (12).

The ratio of the half diagonal of the base of the multilateral needles(12) and (13) to the height of the pyramid is at least one to five. Thesurfaces of the multilateral needles (12) and the surfaces of themultilateral central needle (13) have a total surface more than seventimes bigger than that of the surface (9). This reflects approximatelythe internal surface of the essence of the bone bark on which the headis implanted. Another such advantageous surface constitutes the jointsurface of the multilateral needles (4). The different lengths andplaced multilateral needles (4) and (12) causes their steady, easy andsuccessive introduction into the spongy bone to a depth determined bycontact with the bone of the edge (6) and the bearing surface (10).

Insertion of the endoprosthesis of the present invention issimultaneously performed along the axis of the bone's head. The centralneedle (13) of the endoprosthesis's head (2) is introduced into thespongy bone until the bearing surface (10) contacts the cortex and theadjacent spongy trabecular structure, into which are simultaneouslyintroduced multilateral needles (12). Needles (12) are introduced intothat part of the bone which is at a distance larger than that of theplane of the bearing surface (10), and the space between the remainingneedle (12) which have not been introduced into the spongy bone untilthe bearing surface (11) is filled up `in "biological silence" byosteoblasts.

The glenoid cavity of the endoprosthesis is implanted according to themethod of this invention so that the edge (6) is brought into contactwith the bone, and is positioned on the bone perpendicular to thecentral axis of the acetabulum of the bone. The edge (6) is placedsymmetrically with regard to the axis. The needles which are near thespherical belt on the boundary surface of the endoprosthesis aresimultaneously partially introduced into the spongy structure of thebone. The remaining surface between the multilateral needles (4) whichare not introduced into the spongy bone down to the boundary surface (3)are filled by osteoblasts in "biological silence".

The bearing edge (6) as well as the bearing surface (10) properly bearon the bone of the pelvis acetabulum and on the bark edge of the bonetight femoral neck. The needles' proportions, according to presentinvention, considerably increases the adhesion when bearing on thespongy bone, which causes a more effective absorption of forces, andmoreover increases the fixation power in the bone tissue, thuspreventing the endoprosthesis from spraining and loosening.

After new bone formation, the boundary surface (3) of the acetabulum(1), the boundary surface (11) of the head (2), surface (5), bearingsurface (10) and the surfaces of the multilateral needles (4), (12), and(13) become the bearing surfaces of the endoprosthesis.

The endoprosthesis is produced in various dimensions properly fixedaccording to the gradation of biological sizes, and from biologicallynon-toxic materials, and is possessing the proper mechanical properties.The acetabulum as wen as the head can be produced entirely out ofplastics with a varying density gradation so that its resistance powerwould be a function of the varying spring energy of the plastic,decreasing in due measure, as it removes from the surface in both partsof the endoprosthesis which achieves a suppression of the poweractivities which occur in the joint's system.

One of the advantages of the endoprosthesis is the proper arrangement ofpower between the bone and the endoprosthesis. Moreover, theimplantation operation is simplified and can be performed with sickpeople in a considerably wider age range.

Another advantage is the longer durability of the endoprosthesis and, ifnecessary, it is easy to replace because the endoprosthesis in its shapeis always very close to the biological model. At the same time, theendoprosthesis facilitates its application with patients with severeosteoporosis and enables implantation without using surgical cement.

Also it should be emphasized that endoprosthesis implantation accordingto this invention causes less injuries to the bone tissues.

The endoprosthesis in biologically verified dimensions according to thisinvention can be applied in operations of bone surgery on all sphericaljoints which occur with people and in animals and especially withimplantations of hip joints, elbow joints, knee joints, shoulder joints,ankle bone joints, hand and foot joints as well as with implantations ofinter-vertebra discs. The subject of this invention can be applied alsoin all other cases of similar biomechanical properties. While theinvention has been described in detail with reference to preferredembodiments thereof, it will be apparent to one skilled in the art thatvarious changes can be made, and equivalents employed, without departingfrom the scope of the invention.

I claim:
 1. An endoprosthesis for implantation in a joint, comprising:anacetabulum prosthesis comprising a cap having an exterior surface and aninterior surface, a plurality of substantially parallel cap needlesextending from said cap exterior surface, and a cap axis; and aprosthetic head comprising an exterior surface sized to fit within saidcap interior surface, an interior surface, a plurality of substantiallyparallel head needles extending from said head interior surface, and ahead axis; wherein a total surface area of at least one of all of saidcap needles and all of said head needles is at least seven times greaterthan a total surface area of said head exterior surface.
 2. The jointendoprosthesis according to claim 1, wherein said cap exterior surfaceis substantially hemispherical, said head exterior surface is shaped asa portion of a sphere, and said head interior surface is substantiallyhemispherical.
 3. The joint endoprosthesis according to claim 1, whereineach of said cap needles is multilateral.
 4. The joint endoprosthesisaccording to claim 3, wherein each of said cap needles is pyramidal. 5.The joint endoprosthesis according to claim 1, wherein each of said capneedles comprises a base, and the bases of adjacent cap needles contacteach other.
 6. The joint endoprosthesis according to claim 1, whereinthe ratio of one-half of the largest cross-sectional dimension to theheight of each said cap needle is at least 1:5.
 7. The jointendoprosthesis according to claim 1, wherein said cap exterior surfacecomprises a central planar bearing surface substantially perpendicularto said cap axis.
 8. The joint endoprosthesis according to claim 7,wherein said cap needles comprise needles immediately adjacent said capcentral planar bearing surface shorter than said cap needles.
 9. Thejoint endoprosthesis according to claim 1, wherein a total surface areaof all of said cap needles is at least seven times greater than a totalsurface area of said head exterior surface.
 10. The joint endoprosthesisaccording to claim 1, wherein said cap needles each comprise a needlelength and a needle end, said cap needle lengths being selected suchthat said cap needle ends are all located on one side of a planecoincident with said cap bearing surface.
 11. The joint endoprosthesisaccording to claim 1, wherein each of said head needles is multilateral.12. The joint endoprosthesis according to claim 11, wherein each of saidhead needles in pyramidal.
 13. The joint endoprosthesis according toclaim 1, wherein each of said head needles comprises a base, and thebases of adjacent head needles contact each other.
 14. The jointendoprosthesis according to claim 1, wherein the ratio of one-half ofthe largest cross-sectional dimension to the height of each said headneedle is at least 1:5.
 15. The joint endoprosthesis according to claim1, wherein said head exterior surface comprises a peripheral planarbearing surface substantially perpendicular to said head axis.
 16. Thejoint endoprosthesis according to claim 5, wherein said head needlescomprise needles immediately adjacent said head peripheral planarbearing surface shorter than said head needles.
 17. The jointendoprosthesis according to claim 1, wherein said head needles comprisea central needle extending from said head interior surface which islonger than the remaining head needles.
 18. The joint endoprosthesisaccording to claim 1, wherein a total surface area of all of said headneedles is at least seven times greater than a total surface area ofsaid head exterior surface.
 19. The joint endoprosthesis according toclaim 1, wherein said head bearing surface is substantially annular,including an external diameter less than a diameter of said headexterior surface.
 20. The joint endoprosthesis according to claim 1,wherein a total surface area of said cap needles and a total surfacearea of said head needles are both at least seven times greater than atotal surface area of said head exterior surface.